Catalysts for use in automotive appliances, such as for catalyzing exhaust fumes of combustion engines or the like, comprise a body having a plurality of inner channels through which the exhaust gas is made to flow. The inner surfaces of the channels are coated with a catalytic material which catalytically supports a chemical reaction of components of the exhaust fumes as being well-known in the art.
In manufacturing of such catalysts, the inner surfaces of channels in a workpiece are coated with a coating medium, such as a washcoat and/or a coating slurry containing the corresponding catalytic material. The inner surfaces are brought into contact with the coating medium so that a layer of the coating medium is applied onto the inner surfaces of the body. Depending on the kind of coating process, the height varies up to which the coating medium shall be introduced into the channels. As the height of the coating medium within the channel cannot be accurately measured, equally applying the coating medium on a predetermined portion of the length of the channels is difficult.
Conventional approaches provide to bring the openings of one side of the substrate into contact with the coating medium and to draw the liquid coating medium through the channels of the substrate by applying a vacuum to the opposite side of the substrate. By means of a predetermined pressure, the coating may be generally applied onto a portion of the length of the channels.
If the coating medium is brought into the channels of the workpiece by pressure from below, then level sensors may be used to obtain an indication of a reference level. The reference level together with a predetermined position of the workpiece may define the total filling volume between the reference level and the liquid coating level up to which the coating shall be applied onto the inner surfaces of the substrate.
However, the channels through the substrate usually have a low cross-section, the total filling volume is also low and therefore a dosing unit has to be applied, as coating medium pumps cannot provide the required accuracy. Although dosing units may be suitable to accurately provide a predetermined amount of coating medium into a coating unit, the flow rate of the coating medium provided by the dosing unit is slow, so that the overall cycle time for coating a workpiece is high.
On the other hand, accurately dispensing of a predetermined amount of coating medium by adjusting a pumping rate of a coating medium pump or by switching respective valves is too inaccurate with respect to the low inner volumes of the workpieces to be filled.
It is therefore an object of the present invention to provide a method and apparatus for partly coating an inner surface of a workpiece which provides a low cycle time and a high accuracy.